Polysulfone resin has been extensively applied and developed as a medical material because of its excellent heat resistance, chemical resistance and .gamma.-ray resistance. The polysulfone resin is also used as a material in highly transmissive artificial dialyzers. However, the polysulfone itself is hydrophobic and exhibits poor blood compatibility by itself. Hitherto, various methods have been developed in an attempt to improve the compatibility with the blood. For example, Japanese Unexamined Patent Publication (Kokai) No. 61-93801 discloses a method of adding polyvinylpyrrolidone to thereby improve the blood compatibility of the membrane and Japanese Unexamined Patent Publication (Kokai) No. 6-165926 discloses a polysulfone hollow fiber membrane containing a vinylpyrrolidone-base polymer and a polyglycol.
The compatibility with the blood can be improved by blending a hydrophilic polymer as in these techniques, however, since the hydrophilic polymer blended with the polysulfone resin is water-soluble, a thorough washing of the membrane formed is essential. Due to this, the washing step generally takes a long time and the film formation process is inefficient. Further, when a water-soluble hydrophilic polymer is added, in addition to the problem of a cumbersome washing process, there is a serious problem in the production, due to the fact that the water-soluble polymer added during the film formation is eluted in a large amount into the coagulating solution. More specifically, at the time of recovering a solvent of the membrane from the coagulating solution, the solvent becomes difficult to recover because the viscosity of the coagulating solution is greatly increased due to the presence of the hydrophilic polymer.
From the standpoint of suppressing elution of the added hydrophilic polymer, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 63-97205 and 4-300636 disclose a technique of subjecting a polysulfone-base membrane having added thereto a hydrophilic polymer such as polyvinylpyrrolidone, to heat treatment or radiation treatment. However, the heat treatment must be performed at a fairly high temperature (170.degree. C. or more) and the membrane performance is difficult to maintain. Further, the method of effecting cross-linking by high-power .gamma.-ray irradiation or the like may reduce the blood compatibility of the membrane. Furthermore, these methods cannot overcome the problem accompanying the elution of the hydrophilic polymer into the coagulating solution.
In order to improve the water permeability of the polysulfone membrane, a hydrophilic polymer having low solubility in water may be added. In this respect, a method for forming a membrane comprising adding a graft copolymer or block copolymer consisting of a polysulfone segment and a hydrophilic polymer segment is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 62-168503, 62-199621, 62-201603, 63-88003, 63-84603 and 2-140234.